Induction logging signals and antenna systems

ABSTRACT

An apparatus for estimating a property of an earth formation penetrated by a borehole includes: a carrier configured to be conveyed through the borehole; a transmitter antenna disposed at the carrier and configured to emit electromagnetic energy into the earth formation; a controller configured to control electrical current of frequency f transmitted to the transmitter antenna, wherein the transmitted electrical current is non-sinusoidal having a first section with a uniform positive slope and a second section with a uniform negative slope; a receiver antenna configured to receive a signal from the formation indicative of the property; and a processor configured to receive the signal from the receiver antenna and to estimate the property using the received signal.

BACKGROUND

Geologic formations may be used for various purposes such as hydrocarbonproduction, geothermal production, and carbon dioxide sequestration. Sothat resources devoted to these purposes may be efficiently used, it isimportant to characterize the formations. Typically, many differenttypes of tools and instruments may be disposed in boreholes penetratingthe formations in order to characterize or determine properties of theformations.

One type of tool used to characterize formations is an induction loggingtool. The induction logging tool induces electrical currents in aformation of interest and receives signals in response to the inducedcurrent. The signals include information related to an electricalcharacteristic of the formation of interest such as the formation'sresistivity or its inverse conductivity. Because electricalcharacteristics can vary throughout the formation, improvements toinduction logging tools that increase or improve their sensitivity wouldbe appreciated by the drilling industry.

BRIEF SUMMARY

Disclosed is an apparatus for estimating a property of an earthformation penetrated by a borehole. The apparatus includes: a carrierconfigured to be conveyed through the borehole; a transmitter antennadisposed at the carrier and configured to emit electromagnetic energyinto the earth formation; a controller configured to control electricalcurrent of frequency f transmitted to the transmitter antenna, whereinthe transmitted electrical current is non-sinusoidal having a firstsection with a uniform positive slope and a second section with auniform negative slope; a receiver antenna configured to receive asignal from the formation indicative of the property; and a processorconfigured to receive the signal from the receiver antenna and toestimate the property using the received signal.

Also disclosed is a method for estimating a property of a formationpenetrated by a borehole. The method includes: conveying a carrierthrough the borehole; transmitting electrical current at frequency f toa transmitter antenna disposed at the carrier in order to emitelectromagnetic energy into the earth formation, wherein a controllercontrols the electrical current such that the electrical current isnon-sinusoidal having a first section with a uniform positive slope anda second section with a uniform negative slope; receiving a signal fromthe formation indicative of the property using a receiver antenna; andestimating the property using the received signal.

Further disclosed is a non-transitory computer readable medium havingcomputer executable instructions for estimating a property of aformation penetrated by a borehole by implementing a method thatincludes: transmitting electrical current to a transmitter antennadisposed at the carrier in order to emit electromagnetic energy offrequency f into the earth formation, wherein a controller controls theelectrical current such that the electrical current is non-sinusoidalhaving a first section with a uniform positive slope and a secondsection with a uniform negative slope; receiving a signal from theformation indicative of the property using a receiver antenna; andestimating the property using the received signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The following descriptions should not be considered limiting in any way.With reference to the accompanying drawings, like elements are numberedalike:

FIG. 1 illustrates an exemplary embodiment of a downhole tool disposedin a borehole penetrating the earth;

FIG. 2 depicts aspects of one embodiment of electromagnetic energyemitted by the downhole tool into the earth;

FIG. 3 depicts aspects of another embodiment of electromagnetic energyemitted by the downhole tool into the earth;

FIGS. 4A, 4B, and 4C, collectively referred to as FIG. 4, depict aspectsof various embodiments of electromagnetic energy emitted by the downholetool into the earth;

FIG. 5 illustrates a flow chart for a method for estimating a propertyof an earth formation penetrated by a borehole;

FIG. 6 depicts aspects of an antenna assembly and a transceiver used inpresenting various antenna configuration embodiments;

FIG. 7 depicts aspects of two antenna assemblies in the downhole tool ina deviated borehole;

FIG. 8 depicts aspects of a single antenna assembly in the downhole toolin a vertical borehole;

FIG. 9 depicts aspects of antenna assemblies having a spiraling screwshape for focusing induction currents;

FIG. 10 depicts aspects of antenna assemblies provide two spiralingscrew shapes for longer coils and lower frequencies;

FIG. 11 depicts aspects of antenna system orientations for shallow anddeep investigation of earth formations;

FIG. 12 depicts aspects of antenna system orientations directionallyorientated investigations of earth formations;

FIG. 13 depicts aspects of selecting combinations of antenna systems forinvestigating various depths into earth formations;

FIG. 14 depicts aspects of antenna system orientations for lookingforward ahead of a drill bit;

FIG. 15 depicts aspects of antenna combinations to increaseinvestigation depths using circular induction currents;

FIG. 16 depicts aspects of horn waveguides in an induction logging tool;

FIG. 17 depicts aspects of an omni-directional horn waveguide in aninduction logging tool; and

FIG. 18 illustrates a flow chart for another method for estimating aproperty of an earth formation penetrated by a borehole.

DETAILED DESCRIPTION

A detailed description of one or more embodiments of the disclosedapparatus and method presented herein by way of exemplification and notlimitation with reference to the Figures.

FIG. 1 illustrates a cross-sectional view of an exemplary embodiment ofan induction logging tool 10 disposed in a borehole 2 penetrating theearth 3, which includes an earth formation 4. The formation 4 representsany subsurface material of interest (including borehole material andinvasion zone). The induction logging tool 10 is conveyed through theborehole 2 by a carrier 5. In the embodiment of FIG. 1, the carrier 5 isa drill string 6 in an embodiment known as logging-while-drilling (LWD).Disposed at a distal end of the drill string 6 is a drill bit 7. Adrilling rig 8 is configured to conduct drilling operations such asrotating the drill string 6 and thus the drill bit 7 in order to drillthe borehole 2. In addition, the drilling rig 8 is configured to pumpdrilling fluid through the drill string 6 in order to lubricate thedrill bit 7 and flush cuttings from the borehole 2. Downhole electronics9 are configured to operate the induction logging tool 10, processmeasurements or data received from the tool 10, record the data forlater retrieval, and/or provide a telemetry interface. Telemetry is usedto provide communications between the induction logging tool 10 and acomputer processing system 11 disposed at the surface of the earth 3.Data processing or tool operations can also be performed by the computerprocessing system 11 in addition to or in lieu of the downholeelectronics 9. The induction logging tool 10 may operate intermittently,at particular intervals, or continuously during the drilling process. Inan alternative embodiment, the carrier 5 can be an armored wireline inan embodiment known as wireline logging. Other conveyances can bepumping down an induction tool through drill pipe or attaching aninduction tool to coiled tubing lowered into the earth.

The induction logging tool (ILT) 10 is configured to performmeasurements of properties of the formation 4. Non-limiting examples ofthe properties include resistivity, conductivity, fracture patterndetection and location, or fault pattern detection and location. The ILT10 includes a transmitter 14 coupled to a transmitter antenna 13. Thetransmitter 14 is configured to apply voltage or current at amplitude Aand frequency f to the transmitter antenna 13, which is configured toemit electromagnetic energy having the frequency f. The emittedelectromagnetic energy induces electrical currents, which may bereferred to as induced currents, eddy currents or Foucault currents, inthe formation 4. The induced currents in turn emit electromagneticenergy also referred to as signals having a characteristic related to anelectrical property of interest of the formation 4. Hence, by measuringthe signals to determine the characteristic, the electrical property ofinterest can be determined.

In order to receive the return signals, the ILT 10 includes a receiverantenna 15 coupled to a receiver 16. The receiver antenna 15 isconfigured to receive the signals and to convert them to electricalsignals that are amplified by the receiver 15. The electrical signalsare characterized by the downhole electronics 9 or the computerprocessing system 11 in order to determine or estimate the property ofinterest. Received electrical signals for specific probed formationregions may be derived from complex current related patterns and may bereferred to as signatures because of their unique identity.

It can be appreciated that the transmitter antenna 13 and the receiverantenna 15 can have various embodiments. In one or more embodiments, oneor more of the antennas 13 and 15 can be coils. It can also beappreciated that transmitting and receiving function of the antennas 13and 15 can be incorporated into a single antenna or group of antennaswhere each antenna can be configured to both transmit and receiveelectromagnetic energy. Hence, discussions related to transmitting orreceiving electromagnetic energy may inherently include antennas thatboth transmit and receive electromagnetic energy and functions of thetransmitter 14 and the receiver 16 can be included in a transceiver.Similarly, discussions related to a transceiver and transmitting andreceiving (i.e., transceiver) antennas may inherently include usingseparate transmitters and receivers and separate transmitting andreceiving antennas.

A controller 19 is coupled to the transmitter 14 and the receiver 16 andis configured to control the output of the transmitter 14 based on inputreceived from the receiver 16. Non-limiting embodiments of aspects ofthe electromagnetic energy emitted by the transmitter antenna 13 includeamplitude, frequency, slope of a wave form, and duration of the slope.In one or more embodiments, the transmitter 13 includes adigital-to-analog converter (DAC) 17 configured to convert a digitalsignal received from the controller 14 into an analog signal or waveform having a desired characteristic for transmission into the formation4. Similarly, in one or more embodiments, the receiver 16 includes ananalog-to-digital converter (ADC) 18 configured to convert an analogsignal received from the formation 4 into a digital signal forprocessing. It can be appreciated that various functions of the downholeelectronics 9, the computer processing system 11, the transmitter 14,the receiver 16, and the controller 19 may be performed by any one ofthese devices or distributed among two or more of these devices.

FIG. 2 depicts aspects of sinusoidal current in a transmitter coil. At21, high rates of change of coil current strongly induce formationcurrents, while lower and flattening rates of change of coil current at22 weakly induce formation currents within the duty cycle shown.Portions of sine wave signals have rapidly varying currents and otherportions of the sine wave signals have very slowly changing currents. Asa consequence, currents induced in formations by sine waves can varygreatly or attenuate over significant portions of wave time periods.Also, the high rates of change in sine wave currents frequently overlyinfluence shallower parts of formations near well bores invaded bydrilling fluids. In contrast, the lower rates of current changes nearwave peaks and troughs may have some abilities to penetrate to greaterradial depths or distances from the borehole. Unfortunately, weakerinduction strength capabilities may often limit penetration depths dueto smaller amplitudes of induced signals at greater depths. Weaker deepsignals are more difficult to detect and frequently have lower signal tonoise ratios.

In order to provide stronger deep signals, the transmitter 14 and thetransmitter antenna 13 in one or more embodiments are configured to emitelectromagnetic energy in a waveform having a “saw-tooth” pattern 30 asillustrated in FIG. 3. Longer ramped current pulses can at first induceshallow eddy currents and a progression of currents more deeply withinformations. At 31, the coil current induces an early shallow depthresponse from the formation 4. At 32, progressively longer and deeperformation responses are induced at later times. At 33, a rapid coilcurrent change may induce more shallow responses (as opposed to deeperresponses if coil current change occurs over a longer duration) from theformation 4. At 34, a region of sign-change from positive slope tonegative slope occurs more rapidly than in a sine wave. Onecharacteristic of the saw-tooth pattern 30 is that the time duration forthe current change (i.e., at the corners) from maximum magnitudepositive slope (or maximum magnitude negative slope) to maximummagnitude negative slope (or maximum magnitude positive slope)illustrated at 34 is less than those current changes in a sine wave. Tobe clear, the time duration for those current changes in a sine wave isone-half the time period of that sine wave (i.e., timeduration=1/(2·frequency). Magnitude of a slope refers to the absolutevalue of the slope. The saw-tooth pattern may also be described ashaving a first section 35 with a uniform positive slope and a secondsection 36 with a uniform negative slope. In one or more embodiments,the controller 19 may be configured to generate pulses of the emittedelectromagnetic energy with curving or increasing or decreasing rates ofchange of currents to cancel systematic inductance or capacitancecharacteristics of the circuitry, the borehole 2, or various portions ofthe formation 4. Received signals can be processed to dynamically altercontroller outputs to generate variations in more emittedelectromagnetic energy capable of inducing currents with characteristicsthat will compensate for and better measure localized borehole andformation characteristics.

FIG. 4 depicts aspects of various saw-tooth patterns of coil currentsfor transmitting electromagnetic energy in to the formation 4. In FIG.4A, the saw-tooth pattern is symmetrical with the absolute value of thepositive slope at 41 equal to the absolute value of the negative slopeat 42. In FIG. 4B, the saw-tooth pattern is asymmetrical with theabsolute value of the positive slope at 43 being less than the absolutevalue of the negative slope at 44. In FIG. 4C, the saw-tooth pattern issymmetrical with the absolute value of the positive slope at 45 beinggreater than the absolute value of the negative slope at 46. Generally,the saw-tooth pattern excludes square wave type pulses having verticalslopes that are undefined as division by zero.

As noted above, the controller 19 may be configured to control theoutput of the transmitter 14 based on input received from the receiver16 due to receiving a return signal from the formation 4. For example,the induction logging tool 10 may emit first electromagnetic energyhaving a first characteristic that results in the receiver antenna 15receiving a return signal from the formation 4. The return signalincludes information that the controller 19 may use to emit secondelectromagnetic energy into the formation 4. The second electromagneticenergy has a second characteristic, which is determined by thecontroller 19 using a characteristic of the return signal. Thecontroller 19 may determine the second characteristic using a look-uptable or by implementing an algorithm that acts upon the characteristicof the return signal. It can be appreciated that by changing the secondcharacteristic, the ILT 10 may probe deeper into the formation orreceive a return signal having a higher signal to noise ratio. Forexample, if the return signal indicates a deeper invasion zone (i.e.,zone near borehole infiltrated by drilling fluid) than expected, thenthe controller 19 can signal the transmitter 14 to decrease thefrequency f of the transmitter current such that the transmitter current(and thus the emitted electromagnetic energy) has an increased ramp-uptime to probe deeper into the formation. For another example, if thinbeds are initially detected, then the controller 19 can signal thetransmitter 14 to increase the frequency f to have a greater rate ofchange of uniform slope (positive slope and/or negative slope) of thetransmitter current to better measure the thin beds. In a furtherexample, in very resistive beds, the controller 19 can signal thetransmitter 14 to transmit lower amplitude current in order to preventhigher amplitude induced currents from reaching more conductive upper orlower beds. It can be appreciated that combinations of stronger and/orweaker induced currents may be induced by using different antennasystems in the ILT 10 to produce combinations of currents (and relatedsignals) that can accommodate complex variations in clusters ofdifferent beds with wide ranges of conductivities. It can also beappreciated that the controller 19, using reactive controller software,can respond to rapidly changing characteristics of various formationsegments or regions to more accurately characterize variations withinthe earth with higher resolution than prior art logging systems.

Using digital to analog drivers to produce many variations intransmitter currents within coils in groups can result in many usefulcombinations of induced formation currents having complex flow pathshapes. Abundant measurements expedite mathematical sorting and analysisfor deriving more localized formation characteristics.

FIG. 5 illustrates a flow chart for a method 50 for estimating aproperty of an earth formation penetrated by a borehole. Block 51 callsfor conveying a carrier through the borehole. Block 52 calls fortransmitting electrical current at frequency f to a transmitter antennadisposed at the carrier in order to emit electromagnetic energy into theearth formation, wherein a controller controls the electrical currentsuch that the electrical current is non-sinusoidal having a firstsection with a uniform positive slope and a second section with auniform negative slope. Block 53 calls for receiving a signal from theformation indicative of the property using a receiver antenna. Block 54calls for estimating the property with a processor that receives thesignal from the receiver antenna. The method 50 may also includeemitting other electromagnetic energy having a characteristic determinedby the controller based on the received signal, receiving another signaldue to the emitting of the other electromagnetic energy, and estimatingthe property using the another signal. The method 50 may also includetransmitting the electrical current to the transmitter antenna as pulsesof electrical current having the frequency f where a pulse repetitionrate may be varied and/or the frequency f of electrical current in eachpulse may be varied for different pulses. The method 50 may alsoinclude, in addition to the transmitting of non-sinusoidal electricalcurrent, transmitting sinusoidal electrical current to the transmitterantenna to provide measurements similar to conventional logging toolsfor classical reference data to compare with past measurements inpreviously logged formations.

Next, various embodiments of antenna systems are introduced. Theseantenna systems provide focusing surfaces such as antenna groups andwaveguides that may emit, induce, receive, select, and concentrateelectromagnetic energy or signals. For convenience and clarity, FIG. 6illustrates one example of an antenna system 60 that is referred to insome of the following antenna system embodiments. While not shown forclarity purposes, the antenna system may be disposed at the ILT 10 orcarrier 5. The term “disposed at” relates to the antenna system 60 beingdisposed on, in, within or coupled to ILT 10 or carrier 5. The antennasystem 60 includes a transceiver antenna 61 configured to transmitand/or receive electromagnetic energy or signals. A transceiver 64 iscoupled to the transceiver antenna 61 and is configured to transmit orreceive electrical current to or from the transceiver antenna 61. Theantenna system 60 also includes a reflector antenna 62 configured toreflect electromagnetic energy transmitted by the transceiver antenna 61or to reflect signals received from the formation 4 to the transceiverantenna 61. A structure 63 such as a bracket may be used to support andmaintain alignment of the antennas 61 and 62. Articulating supportsdriven by electrical motors may also move or reorient the antennas. Itcan be appreciated that functions of the transceiver 64 may be performedby separate devices such as the transmitter 14 and the receiver 16.Similarly, functions of the transceiver antenna 61 may be performed byseparate antennas such as the transmitter antenna 13 and the receiverantenna 15.

In general, logging tools are long but very narrow in order to beconveyed through the borehole. Standard antenna shapes such asparabolics are often quite large and unsuited for narrow boreholes.Fortunately, fully radial parabolics or other shapes are not necessaryto transmit, gather, or concentrate electromagnetic signals. Longer andlarger but narrow antenna shapes may be used when combined with smallcurved secondary antenna groups as illustrated in FIGS. 7 and 8 forexample. The actual antenna curvature will depend upon physicalconstraints such as the inside diameter available within specificlogging tools. Somewhat angled antenna orientations with respect to thetool length axis further allow for still longer antenna geometries,thus, providing for even lower frequencies or longer shaped pulses thanthose used in conventional logging tools.

In one or more embodiments, the ILT 10 includes a spiral “screw” likeshape with small (i.e., small enough to fit within ILT 10) parabolic“grooving” 90 in order to focus or concentrate signals radiating out ofa spiraling coiled wire 91 wound along the focal spacings as illustratedin FIG. 9. Thus, the usual large signal spreading of conventionalinduction tool coils is avoided. Short duty cycle high energy currentpulses through the wire 91 may allow for relatively stronger signals topenetrate deeper into formations without overheating of circuitry. Also,the parabolic shapes can accommodate variations in frequencies forelectromagnetic waves and variable grouped pulse shapes. Back lobeemissions may be absorbed by electromagnetic emission absorbing material92 such as steel wool, copper alloys, or other conductive materials.Receiving parabolic screw antenna shapes may respond far less tospurious shoulder bed signals than conventional coil systems.

Half or partial rather than full parabolic antenna systems may allow formore compact logging tools and thinner bed resolutions due to providingan increased number of antenna systems 60 in the ILT 10 as illustratedin FIGS. 10-13. In FIG. 10, the ILT 10 includes as spiral shaped half orpartial grooving 100 for reflecting electromagnetic energy or signalsfrom or to the spiral coiled wire 101 (acting as transmitting orreceiving antenna). In one or more embodiments, the grooving 100 isconfigured such that a focus of a section of the grooving 100 overlapsthe focus of another section of grooving 100 in order to provide fordeep investigation orientations and to help limit counter currentspreading in the formation 4. In one or more embodiments, two wires (101and 102) are alternately wrapped in the grooving 100 to provide twooverlapping spiraling screw shapes for longer coils and lowerfrequencies f.

Numerous angular orientations may be used for antenna combinations toinvestigate different depths (both radial and longitudinal) and portionsof formations as illustrated in FIGS. 11-14.

In the side-view of FIG. 11, one or more of the antenna systems 60 canbe angled for shallow or deep investigation orientations that includeboth horizontal and vertical components for anisotropy sorting. In oneor more embodiments, a pair of antenna systems 60 is configured tooverlap a volume of investigation of another pair of antenna systems 60.In addition, a motor 110 may be coupled to one or more of the antennasystems 60 in order to provide scanning at different angles. In one ormore embodiments, the motor 110 may be controlled by the controller 19in order to coordinate setting the angle with a specific characteristicof the electrical current to be transmitted to the transceiver antenna61. In one or more embodiments, the motor 110 may be configured to movethe antenna system or individual antennas in the antenna system such asby displacement or rotation.

In the top-view of FIG. 12, a group of antenna systems 60 areazimuthally distributed about a circumference of the ILT 10 in order toobtain azimuthal measurements that provide formation directionalanisotropies. The group of antenna systems 60 illustrated in FIG. 12 canbe used alone or in combination with other antenna systems such as thoseshown in the other figures herein.

In the side-view of FIG. 13, the antenna systems 60 have differentangles of orientation. By having the controller 19 select a combinationof certain antenna systems 60 having specific orientation angles,various depths into the formation 4 may be probed. In one or moreembodiments, the controller 19 may select an antenna system 60 toalternately emit electromagnetic energy that results in a signal beingreceived by another antenna system 60 and then receive signals due tothe emission of electromagnetic energy by the another antenna system 60.

In the side view of FIG. 14, the antenna systems 60 have orientationangles for probing ahead of the ILT 10 in the borehole 2. As in theembodiment of FIG. 13, the antenna systems 60 may alternate transmittingand receiving functions. In one or more embodiments, the look-aheadmeasurements may be transmitted to the computer processing system 11,which may use the measurements to input control signals to the drillingrig 8 in order to steer the drilling of the borehole 2.

Antennas systems 60 with parabolic, partial parabolic, or variousaltered curvatures can acquire induced formation signals fromelectromagnetic energy emitted by a vertical or otherwise oriented coil150 as illustrated in the side-view of FIG. 15. The embodiment of FIG.15 provides focused reception of localized portions of circularinduction current shown at 151, which may be horizontal as well asvertical. Advantages include sampling specific, oriented, and localizedportions of the total induction signals from within formations. Also,signature signals may be acquired by rapid sampling using the analog todigital converters, especially if a variety of signals are beinggenerated by the oriented transmitter coils.

It can be appreciated that a waveguide such as a horn waveguide or hornantenna, as illustrated in FIG. 16, may be used in lieu of or inaddition to a reflector antenna. A horn waveguide 160 or horn antennagroups can be placed at many varying orientations and can be rotated orreoriented by the motor 110. Also, horn antenna systems can be focusedor oriented toward selected parts of formations. Information aboutformation dips (or slopes) may be extractible from combined processingof signature patterns that vary with different orientations and depthsof investigations. Angled horn orientations allow for slightly longerantenna sizes within tools. Also, a horn major semi axis can be orientedalong the tool length with the shorter minor semi axis across therestricted width of the tool. Hence, larger horn signal entrance or exitshapes and larger total areas for waveguide type cross sections becomepossible. Fully circular ‘Omni’ horn antennas, as illustrated in FIG.17, can provide radial coverage rather than localized focusing ororientations.

Processing combinations of signals from many antenna groupings mayenable sorting of formation areas contributing to composite signals.Angular localized conductivity data may be used in making betterconductivity and implied resistivity corrections for laminated or thinlyinterbedded zones. Current flaring into more conductive thin layers maybe more detectable with varied combinations of signals and measurementsthan with conventional coils and sine waves. Current flaring aroundhigher resistivity hydrocarbon bearing zones (and thin layers) oftenleads to underestimating the volume of hydrocarbons within reservoirsand can, thus, be avoided by the present disclosure.

FIG. 18 is a flow chart illustrating a method 180 for estimating aproperty of a formation penetrated by a borehole. Block 181 calls forconveying a carrier through the borehole. Block 182 calls for emittingelectromagnetic energy using a transmitter antenna. Block 183 calls fordirecting the electromagnetic energy into the formation using adirectional guidance antenna. Block 184 calls for receiving signals fromthe formation indicative of the property using the directional guidanceantenna. Block 185 calls for receiving the signals from the directionalguidance antenna using a receiver antenna. Block 186 calls forestimating the property using the received signals.

It can be appreciated that the teachings herein may provide logs havinghighly enhanced induction, dielectric, and electromagnetic signaturepatterns of response signals from earth formations. Detecting fracturesand deriving many more accurate and localized conductivities across arange of distance from boreholes are among the advantages of theseteachings. For example, the teachings can provide borehole fluidinvasion profiles, more accurately derived deep resistivities, andobservable anomalies on signature patterns related to fractures andfaults that lead to valuable evaluations of conventional andunconventional reservoirs. In addition, detected patterns (i.e.,signatures) may have much higher signal to noise ratios compared to morelimited measurements by conventional logging systems.

Further advantages include the ability to detect fractures in many lowpermeability (often shale) reservoirs. Shales, other conductiveminerals, and fluid variations provide paths that often expand ordisperse induced currents into larger geometries. Concurrent increasesin apparent conductivities and decreases in derived resistivities andwater saturations often result. However, more efficient concentrating ofsignals can be accomplished by increasing some focusing coil currents oraltering antenna curvature in order to compress electromagnetic energyand signals into narrower concentrations than in conventional loggingsystems to provide better thin bed resolutions and more specific depthsof investigations.

Further advantages include the abilities of digital to analogtransmitter driver signals to be shaped to counter capacitance,inductance, and other signal distorting characteristics of circuitry,coils, or antenna systems. The digital to analog circuitry may also beused to generate short pulses of electrical current transmitted to thetransmitter antenna 13 with rapid but uniform changes in currents toinvestigate shallow near borehole formation characteristics. Hence, themost appropriately shaped Foucalt, eddy, or induction currents can becreated within formations using the present disclosure.

It can be appreciated that, after transmitting initial electromagneticenergy and receiving initial signals, firmware, software, or algorithmsimplemented by the controller or other processing system, may initiatetransmission of novel and specially altered pulse patterns andfrequencies of the electrical current to specifically evaluatecharacteristics of the borehole, the invasion zone, and/or portions ofgeologic formations based upon detected, derived, or displayed patterns.In addition, the orientations, declinations, and other positioning ofthe antennas can be specifically and dynamically altered in many variedcombinations to more effectively investigate portions of the earth andborehole after receiving and evaluating the initial signals.

It can be appreciated that the advanced antenna systems disclosed hereincan also be designed and size scaled for dielectric well logging thathas been previously done at various high frequencies. Deeper, wider, andlonger investigation signal detections may be obtained from using ‘sawteeth’, or other uniform, or specifically shaped rise times. Moreaccurate measurements from larger formation volumes can be made thanwith conventional sine waves having complex and varying rates of change.Conventional dielectric measurements have severely limited volumes ofinvestigations. Variations in small coil lengths attached to very shorttime staggered digital outputs can be combined to shape generatedsignals with uniformity or specifically shaped rise and fall time rampsor curves for pulses. Many different pulse durations, rise and falltimes, and shapes of pulses may be generated to probe formationcharacteristics. Although common button type dielectric antennacombinations may be used in one or more embodiments, more efficientreceiver antenna curvatures and geometries will capture and concentratestronger detection signals from earth formations. Signals from theantenna systems may be amplified by groups of high frequency cascodepaired transistors with lower intrinsic systematic noises andunilateralized coil compensations for internal capacitances at quitehigh pulse rate changes or high frequencies. However, heavily dopedtunnel diodes acting in high frequency (or fast pulse) quantum avalancheamplification modes or other advanced circuitry may be used asalternatives or in addition to transistors for various signals.Conventional crystal oscillators and signal generation circuits may alsobe incorporated into the induction logging tools for use with theadvanced antenna combinations. Expanding skids and articulating arms(that may include a caliper and micro-type log electrodes or othermeasurement systems) may provide close formation positioning for antennasystems. Amplified formation signals may be sent to very short dutyperiod time staggered analog to digital converters and microprocessorsystems for capturing, recording, and processing of signature patterns.

It can be appreciated that for numerous sets, series, and trains ofpulses, the digital to analog converter can provide specific variationsof uniformly consistent rise and fall rates of current changes.

It can be appreciated that the ILT 10 may be calibrated in a boreholehaving known borehole geometries, known formation geometries, and knownformation electrical properties, in a laboratory with various formationgeometries (including various bed dimensions) and electrical properties,or by analysis.

It can be appreciated that the antennas disclosed herein can havevarious curvatures constrained by various constraints dependent on thedownhole tool size, selected frequency of probing electromagneticenergy, orientation of focus, and focus distance. The curvatures can becalculated using available antenna curvature equations.

In support of the teachings herein, various analysis components may beused, including a digital and/or an analog system. For example, thedownhole electronics 9, the surface computer processing 11, thecontroller 19, the transmitter 14, the receiver 16, the DAC 17 or theADC 18 may include the digital and/or analog system. The system may havecomponents such as a processor, storage media, memory, input, output,communications link (wired, wireless, pulsed mud, optical or other),user interfaces, software programs, signal processors (digital oranalog) and other such components (such as resistors, capacitors,inductors and others) to provide for operation and analyses of theapparatus and methods disclosed herein in any of several mannerswell-appreciated in the art. It is considered that these teachings maybe, but need not be, implemented in conjunction with a set of computerexecutable instructions stored on a non-transitory computer readablemedium, including memory (ROMs, RAMs), optical (CD-ROMs), or magnetic(disks, hard drives), or any other type that when executed causes acomputer to implement the method of the present invention. Theseinstructions may provide for equipment operation, control, datacollection and analysis and other functions deemed relevant by a systemdesigner, owner, user or other such personnel, in addition to thefunctions described in this disclosure.

Further, various other components may be included and called upon forproviding for aspects of the teachings herein. For example, a powersupply (e.g., at least one of a generator, a remote supply and abattery), cooling component, heating component, magnet, electromagnet,sensor, electrode, transmitter, receiver, transceiver, antenna,controller, optical unit, electrical unit or electromechanical unit maybe included in support of the various aspects discussed herein or insupport of other functions beyond this disclosure.

The term “carrier” as used herein means any device, device component,combination of devices, media and/or member that may be used to convey,house, support or otherwise facilitate the use of another device, devicecomponent, combination of devices, media and/or member. Other exemplarynon-limiting carriers include drill strings of the coiled tube type, ofthe jointed pipe type and any combination or portion thereof. Othercarrier examples include casing pipes, wirelines, wireline sondes,slickline sondes, drop shots, bottom-hole-assemblies, drill stringinserts, modules, internal housings and substrate portions thereof.

Elements of the embodiments have been introduced with either thearticles “a” or “an.” The articles are intended to mean that there areone or more of the elements. The terms “including” and “having” areintended to be inclusive such that there may be additional elementsother than the elements listed. The conjunction “or” when used with alist of at least two terms is intended to mean any term or combinationof terms. The terms “first” and “second” are used to distinguishelements and are not used to denote a particular order. The term“couple” relates to coupling a first component to a second componenteither directly or indirectly through an intermediate component. Theterm “directional guidance antenna” relates to a device that isconfigured to redirect incoming electromagnetic energy or signals fromone direction to another direction including a focus to a point, area orvolume. Directional guidance antennas encompass reflector antennas andwaveguides.

It will be recognized that the various components or technologies mayprovide certain necessary or beneficial functionality or features.Accordingly, these functions and features as may be needed in support ofthe appended claims and variations thereof, are recognized as beinginherently included as a part of the teachings herein and a part of theinvention disclosed.

While the invention has been described with reference to exemplaryembodiments, it will be understood that various changes may be made andequivalents may be substituted for elements thereof without departingfrom the scope of the invention. In addition, many modifications will beappreciated to adapt a particular instrument, situation or material tothe teachings of the invention without departing from the essentialscope thereof. Therefore, it is intended that the invention not belimited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

What is claimed is:
 1. An apparatus for estimating a property of anearth formation penetrated by a borehole, the apparatus comprising: acarrier configured to be conveyed through the borehole; a transmitterantenna disposed at the carrier and configured to emit electromagneticenergy into the earth formation; a controller configured to controlelectrical current of frequency f transmitted to the transmitterantenna, wherein the transmitted electrical current is non-sinusoidalhaving a first section with a uniform positive slope and a secondsection with a uniform negative slope; a receiver antenna configured toreceive a signal from the formation indicative of the property; and aprocessor configured to receive the signal from the receiver antenna andto estimate the property using the received signal.
 2. The apparatusaccording to claim 1, wherein the electrical current is transmitted as aseries of pulses.
 3. The apparatus according to claim 2, wherein thecontroller is further configured to vary a shape of one or moredifferent pulses, the frequency f of one or more different pulses, orcombination thereof.
 4. The apparatus according to claim 1, wherein theproperty is resistivity, conductivity, fracture pattern location orfault pattern location.
 5. The apparatus according to claim 1, wherein amagnitude of a slope of the first section is less than a magnitude of aslope of the second section.
 6. The apparatus according to claim 1,wherein a magnitude of a slope of the first section is greater than amagnitude of a slope of the second section.
 7. The apparatus accordingto claim 1, wherein a magnitude of a slope of the positive-slope sideand a magnitude of a slope of the negative-slope side are the same. 8.The apparatus according to claim 1, wherein the controller is furtherconfigured to: transmit first electrical current to the transmitterantenna to emit first electromagnetic energy having a firstcharacteristic into the formation thereby causing the receiver antennato receive a first signal; and transmit second electrical current to thetransmitter antenna to emit second electromagnetic energy having asecond characteristic into the formation thereby causing the receiverantenna to receive a second signal; wherein the controller is furtherconfigured to determine the second characteristic based on the receivedfirst signal and the processor is further configured to estimate theproperty using the second signal.
 9. The apparatus according to claim 8,wherein the second characteristic comprises a change in amplitude, achange in slope of current, or a change in frequency or a change inpulse repetition rate when electrical current is transmitted as a seriesof pulses.
 10. The apparatus according to claim 1, further comprising atransmitter coupled to the transmitter antenna and configured totransmit the non-sinusoidal current to the transmitter antenna in orderto emit the non-sinusoidal electromagnetic energy.
 11. The apparatusaccording to claim 10, wherein the transmitter comprises a digital toanalog converter.
 12. The apparatus according to claim 1, furthercomprising a receiver coupled to the receiver antenna and configured toamplify the signal received by the receiver antenna.
 13. The apparatusaccording to claim 10, wherein the receiver comprises an analog todigital converter.
 14. A method for estimating a property of a formationpenetrated by a borehole, the method comprising: conveying a carrierthrough the borehole; transmitting electrical current at frequency f toa transmitter antenna disposed at the carrier in order to emitelectromagnetic energy into the earth formation, wherein a controllercontrols the electrical current such that the electrical current isnon-sinusoidal having a first section with a uniform positive slope anda second section with a uniform negative slope; receiving a signal fromthe formation indicative of the property using a receiver antenna; andestimating the property using the received signal.
 15. The methodaccording to claim 16, further comprising: transmitting first electricalcurrent having a first characteristic to the transmitter antenna to emitfirst electromagnetic energy into the formation thereby causing thereceiver antenna to receive a first signal; and transmitting secondelectrical current having a second characteristic to the transmitterantenna to emit second electromagnetic energy into the formation therebycausing the receiver antenna to receive a second signal; wherein thecontroller is further configured to determine the second characteristicbased on the received first signal and the processor is furtherconfigured to estimate the property using the second signal.
 16. Themethod according to claim 15, wherein the second characteristiccomprises a change in amplitude, a change in slope of the first section,a change in slope of the second section, or a change in frequency. 17.The method according to claim 15, wherein the first electrical currentis transmitted as a first series of pulses at a first pulse rate and thesecond electrical current is transmitted as a second series of pulses ata second pulse rate and the controller is further configured todetermine the second pulse rate based on the received first signal. 18.The method according to claim 14, wherein the property is resistivity,conductivity, fracture pattern location or fault pattern location 19.The method according to claim 14, wherein a magnitude of a slope of thefirst section is less than a magnitude of a slope of the second section.20. The method according to claim 14, wherein a magnitude of a slope ofthe positive-slope side is greater than a magnitude of a slope of thenegative-slope side.
 21. The method according to claim 14, wherein amagnitude of a slope of the positive-slope side and a magnitude of aslope of the negative-slope side are the same.
 22. A non-transitorycomputer readable medium comprising computer executable instructions forestimating a property of a formation penetrated by a borehole byimplementing a method comprising: transmitting electrical current to atransmitter antenna disposed at the carrier in order to emitelectromagnetic energy of frequency f into the earth formation, whereina controller controls the electrical current such that the electricalcurrent is non-sinusoidal having a first section with a uniform positiveslope and a second section with a uniform negative slope; receiving asignal from the formation indicative of the property using a receiverantenna; and estimating the property using the received signal.
 23. Themedium according to claim 22, wherein transmitting comprisestransmitting first electrical current having a first characteristic tothe transmitter antenna to emit first electromagnetic energy into theformation thereby causing the receiver antenna to receive a first signaland transmitting second electrical current having a secondcharacteristic to the transmitter antenna to emit second electromagneticenergy into the formation thereby causing the receiver antenna toreceive a second signal and the method further comprises: determiningthe second characteristic based on the first signal; and estimating theproperty using the second signal.